1. Field of the Invention
The present invention relates to an apparatus and method for reducing a peak-to-average power ratio (PAPR) in an Orthogonal Frequency Division Multiplexing (OFDM) communication system, and more particularly, to an apparatus and method for reducing a PAPR in an OFDM communication system, in which peaks are reduced below a target power value in an Inverse Fast Fourier Transformation (IFFT) signal when a tone reservation scheme using a gradient algorithm is performed.
2. Background of the Prior Art
In the late 1970's, a cellular mobile telecommunication system was introduced into the United States of America. Since then, a voice communication service was developed in an Advanced Mobile Phone Service (AMPS), known as the 1st Generation (1G) analog mobile telecommunication system. In the mid 1990's, the 2nd Generation (2G) mobile telecommunication system. In the late 1990's, the 3rd Generation (3G) mobile telecommunication system and International Mobile Telecommunication-2000 (IMT-2000) were developed. Both provided advanced wireless multimedia service and a high-speed data service.
Currently, the 3G mobile telecommunication system is evolving to a 4th Generation (4G) mobile telecommunication system. The 4G mobile telecommunication system is currently under the standardization process to integrate service between a wired communication network and a wireless communication network beyond the simple wireless communication service that the previous-generation mobile telecommunication systems provide. In addition, the 4G mobile telecommunication system provides data transmission at higher speeds than the 3G mobile telecommunication system.
When mobile telecommunication systems transmit signals over wireless channels, multi-path interference occurs from various obstacles between the transmitter and the receiver.
The wireless channels where the multi-paths exist are characterized by maximum channel delay spread and signal transmission period. When the signal transmission period is longer than the maximum channel delay spread, interference does not occur in consecutive signals, and frequency characteristics of the channel are given by a frequency nonselective fading.
If a single carrier transmission is used in high-speed data transmission with a short symbol period, an Inter Symbol Interference (ISI) results and distortion increases. Consequently, complexity of a receiver equalizer increases.
To solve the equalization problem in single carrier transmission, the Orthogonal Frequency Division Multiplexing (OFDM) was developed.
The OFDM is a kind of multi-carrier modulation (MCM), in which a serial symbol sequence is converted into parallel symbol sequences and modulated into a plurality of mutually orthogonal sub-carriers, that is, a plurality of sub-carrier channels.
The MCM system was applied to a military high frequency (HF) radio communication for the first time in the late 1950's. The OFDM with overlapping orthogonal sub-carriers was initially developed in the 1970's, but it was difficult to implement the orthogonal modulation between multi-carriers. Therefore, the OFDM had a limitation in practical system implementation. In 1971, Weinstein et al. proposed an OFDM scheme that applies a Discrete Fourier Transform (DFT) to the parallel data transmission as an efficient modulation/demodulation process, which was a driving force behind the development of the OFDM. Also, the use of a guard interval, and a cyclic prefix as a guard interval, further mitigated adverse effects of the multi-path propagation and the delay spread on the systems.
The OFDM has been widely used for digital data communications such as Digital Audio Broadcasting (DAB), digital TV broadcasting, Wireless Local Area Network (WLAN), and Wireless Asynchronous Transfer Mode (WATM). Although hardware complexity was an obstacle to widespread use of OFDM, recent advances in digital signal processing technology, including a Fast Fourier Transform (FFT) and an Inverse Fast Fourier Transform (IFFT), enabled the practical implementation of the OFDM.
The OFDM, similar to a Frequency Division Multiplexing (FDM), provides optimum transmission efficiency in a high-speed data transmission because it transmits data on sub-carriers while maintaining an orthogonality among them. The overlapping use of frequency spectrums is more efficient and is more robust against multi-path fading.
The OFDM reduces the effects of the ISI by use of the guard intervals. In addition, the structure of the equalizer is simplified. OFDM is increasingly popular in communication systems because it is robust against impulse noise.
A conventional OFDM mobile communication system will be described below with reference to FIG. 1, which is a block diagram of a transmitter and a receiver in a conventional OFDM mobile communication system.
Referring to FIG. 1, the transmitter 100 includes a data transmitter 102, an encoder 104, a symbol mapper 106, a serial to parallel converter (SPC) 108, a pilot symbol inserter 110, an IFFT unit 112, a parallel to serial converter (PSC) 114, a guard interval inserter 116, a digital to analog converter (DAC) 118, and an RF processor 120.
In addition, the receiver 150 includes an RF processor 152, an analog to digital converter (ADC) 154, a guard interval remover 156, a serial to parallel converter (SPC) 158, an FFT unit 160, a pilot symbol extractor 162, a channel estimator 164, an equalizer 166, a parallel to serial converter (PSC) 168, a symbol demapper 170, a decoder 172, and a data receiver 174.
In the transmitter 100, the data transmitter 102 generates user data bits and control data bits to be transmitted. The encoder 104 encodes the data bits in accordance with a predetermined coding scheme and outputs the coded bits to the symbol mapper 106. The coding scheme can be, but is not limited to, a turbo coding or a convolutional coding with a predetermined coding rate.
The symbol mapper 106 modulates the coded bits in accordance with a predetermined modulation scheme and produces serial modulation symbols to the serial to parallel converter 108. The modulation scheme can be, but is not limited to, a Binary Phase Shift Keying (BPSK), a Quadrature Phase Shift Keying (QPSK), a 16 Quadrature Amplitude Modulation (16QAM), or a 64 Quadrature Amplitude Modulation (64QAM). The serial to parallel converter 108 converts the serial modulation symbols into parallel modulation symbols, which are inputted to the pilot symbol inserter 110.
The pilot symbol inserter 110 inserts pilot symbols into the parallel modulation symbols and outputs the pilot-inserted parallel modulation symbols to the IFFT unit 112. The IFFT unit 112 performs an N-point IFFT operation on the signals from the pilot symbol inserter 110 and outputs the resultant signals to the parallel to serial converter 114.
The parallel to serial converter 114 performs a parallel to serial conversion on the signals from the IFFT unit 112 and outputs the serial-converted signals to the guard interval inserter 116. The guard interval inserter 116 inserts a guard interval into the serial-converted signals and outputs the guard interval-inserted signals to the digital to analog converter 118. The guard interval is inserted to cancel interference between a previous OFDM symbol and a current OFDM symbol. The guard interval can be inserted in a form of null data having a predetermined interval. In such a case, however, interference between the sub-carriers occurs when a receiver incorrectly estimates a start point of the OFDM symbol. Thus, the probability of misjudging the received OFDM symbol increases. Accordingly, the guard interval is inserted in the form of a cyclic prefix or a cyclic postfix. In the case of a cyclic prefix, a predetermined number of last bits of a time-domain OFDM symbol are copied and inserted before an effective OFDM symbol. In the case of a cyclic postfix, a predetermined number of first bits of a time-domain OFDM symbol are copied and inserted after an effective OFDM symbol.
The digital to analog converter 118 converts the guard interval-inserted signals from the guard interval inserter 116 into analog signals, which are inputted to the RF processor 120. The RF processor 120, including a filter (not shown) and a front end unit (not shown), performs an RF process on the analog signals such that the RF signals can be transmitted through a transmit antenna (Tx antenna) over air. The transmitted signals from the transmitter 150 experience multi-path channels and noisy environments. The transmitted signals are received through a receive antenna (Rx antenna) of the receiver 150.
In the receiver 150, the RF processor 152 down-converts the signals received through the Rx antenna into an intermediate frequency (IF) signals, which are inputted to the analog to digital converter 154. The analog to digital converter 154 converts the analog signals from the RF processor 152 into digital signals, which are inputted to the guard interval remover 156.
The guard interval remover 156 removes the guard interval from the digital signals and outputs the guard interval-removed signals to the serial to parallel converter (SPC) 158. The serial to parallel converter 158 converts the serial signals outputted from the guard interval remover 156 into parallel signals. The FFT unit 160 performs an N-point FFT operation on the parallel signals and outputs the resulting signals to the equalizer 166 and the pilot symbol extractor 162.
The pilot symbol extractor 162 detects pilot symbols from the output signals of the FFT unit 160 and outputs the detected pilot symbols to the channel estimator 164. The channel estimator 164 performs a channel estimation using the pilot symbols outputted from the pilot symbol extractor 162 and transmits the channel estimation result to the equalizer 166. The receiver 150 generates a Channel Quality Information (CQI) corresponding to the channel estimation result and outputs the CQI to the transmitter 100 through a CQI transmitter (not shown).
The equalizer 166 performs channel equalization on the output signals of the FFT unit 160 by using the channel estimation result and outputs the channel-equalized signals to the parallel to serial converter 168. The parallel to serial converter 168 converts the parallel signals from the equalizer 166 into serial signals, and outputs the serial signals to the symbol demapper 170.
The symbol demapper 170 demodulates the serial signals outputted from the parallel to serial converter 168 in accordance with a predetermined demodulation scheme and outputs the demodulated signals to the decoder 172. The decoder 172 decodes the demodulated signals in accordance with a predetermined decoding scheme. Here, the demodulation scheme and the decoding scheme correspond to the modulation scheme and the coding scheme that are applied to the transmitter, respectively.
In spite of the above advantages, the OFDM communication system has a drawback in that it has a high Peak-to-Average Power Ratio (PAPR) due to the multi-carrier modulation. That is, since the OFDM communication system transmits data using multi-carriers, amplitude of the final OFDM signal is the summation of amplitudes of the respective carriers, so the amplitude changes greatly. Further, if all the carriers are in phase, the final OFDM signal will have very large amplitude. As a result, the amplitude of the final OFDM signal is out of a linear dynamic range of a high power amplifier configured within the RF processor 120, making the output signal of the high power amplifier distorted. To obtain a maximum output power, the high power amplifier must operate a device in a nonlinear region. However, due to the distortion, the high power amplifier reduces input power and operates the device in a linear region. This is called a back-off method.
The back-off method decreases the operating point of the high power amplifier to reduce signal distortion. As the back-off value increases, power consumption increases as well, degrading high power amplifier efficiency. The signal with high PAPR degrades the linear amplifier efficiency, and the nonlinear distortion occurs because the operating point is in the nonlinear region. Further, inter-modulation between the carriers and spectrum radiation also result.
In the OFDM communication system, the PAPR can be reduced using a clipping scheme, a block-coding scheme, a phase adjustment scheme, or a tone reservation (TR) scheme.
The clipping scheme determines a predetermined clipping value as a reference value depending on the linear dynamic range of the amplifier and clips the amplitude of a signal above the predetermined clipping value. In the clipping scheme, however, in-band distortion occurs due to nonlinear operation, thus causing inter symbol interference and increasing bit error rate. In addition, channel interference occurs from out-band clipping noise, degrading spectral efficiency.
The block-coding scheme transmits data in a way of adding a coding scheme to spare carriers to reduce the PAPR of all the carriers. The block-coding scheme can correct an error through the coding scheme and reduce the PAPR without any signal distortion. However, when the number of sub-carriers is large, spectral efficiency degrades and the size of a look-up table or generation matrix becomes large. Thus, the complexity and the amount of computation in the block coding scheme increase.
The phase adjustment scheme includes a Partial Transmit Sequence (PTS) scheme and a selective mapping (SLM) scheme.
The PTS scheme performs an L-point IFFT operation on M sub-blocks divided from input data, multiplies the sub-blocks by phase factors to minimize the PAPR, and then transmits data. However, the PTS scheme must perform the IFFT as often as the number (M) of the sub-blocks. Thus, as the number (M) of the sub-blocks increases, the increasing computation of the phase factors prevents high-speed data transmission.
The SLM scheme multiplies M identical data blocks by different phase sequences of a statistically independent N length, and outputs the result with the lowest PAPR. Although the SLM requires M times IFFT operations, it can greatly reduce PAPR and can be used with any number of carriers.
The PTS scheme and the SLM scheme have a drawback in that they must transmit additional information of rotation factors to the receiver to modulate data. In other words, since additional information must be exchanged between the transmitter and the receiver, the communication scheme becomes complex. In addition, when an error occurs in the additional information, an error also occurs in all information of the corresponding OFDM symbol, leading to damage of the OFDM symbol.
The TR scheme that reduces the PAPR by assigning some tones that do not transmit data among the sub-carriers. The receiver ignores the tones that do not transmit information signals, and modulates information signals only from the remaining tones. Thus, the configuration of the receiver can be simplified.
A gradient algorithm is typical among the TR schemes. In the gradient algorithm, the clipping scheme is applied to the TR scheme. A signal having an impulse characteristic is generated using a tone that does not transmit information signal, and an output signal of the IFFT unit is then clipped using the signal having the impulse characteristic. When the output signal of the IFFT unit is clipped using the signal having the impulse characteristic, data distortion occurs only in the tones that do not transmit information signals, while it does not occurs in frequency domain.
The TR scheme using the gradient algorithm will now be described. Herein, the terms “tone” and “sub-carrier” will be used as the same meaning.
FIG. 2 is a block diagram illustrating a structure of a transmitter that employs a conventional TR scheme. Hereinafter, the user data bits and the control data bits, which have been described with reference to FIG. 1, will be referred to as “information signals”. A total of N sub-carriers are divided into L tones to which the information signals are not assigned and (N−L) tones to which the information signals are assigned. In the gradient algorithm, a waveform having an impulse characteristic is produced using the L tones that do not transmit the information signals, and an output signal of the IFFT unit 112 is clipped using the waveform, thereby reducing the PAPR. Hereinafter, the tones to which the information signals are not assigned, that is, the tones to which “null” is assigned, will be referred to as “reserved tones”. The reserved tones are determined by randomly selecting one or more tones among the N tones, performing several hundred thousand to million times the operation of producing the impulse waveform, and combining the tones having the most ideal impulse waveform. In FIG. 2, the reserved tone signals inputted to a tone assignment unit 205 are indicated by dotted lines so as to represent that there are no actually inputted signals.
Referring to FIG. 2, when X(=N−L) information signals 203 are inputted to the tone assignment unit 205, the tone assignment unit 205 assigns the inputted information signals 203 to the remaining tones except the L reserved tones 201. That is, the tone assignment unit 205 outputs the information signals to the previously appointed inputs among the N inputs (positions of the N sub-carriers) of the IFFT unit 207. At this point, no signals are inputted to the positions of the L sub-carriers corresponding to the reserved tones. This relationship can be expressed as Equation (1):
                              X          k                =                  {                                                                                          X                    k                                    ,                                                                              k                  ∉                                      {                                                                  i                        1                                            ,                                              i                        2                                            ,                      …                      ⁢                                                                                          ,                                              i                        L                                                              }                                                                                                                        0                  ,                                                                              k                  ∈                                      {                                                                  i                        1                                            ,                                              i                        2                                            ,                                              …                        ⁢                                                                                                  ⁢                                                  i                          L                                                                                      }                                                                                                          (        1        )            
where, k represents the positions of the sub-carriers, and the set represents the positions of the L sub-carriers corresponding to the reserved tones.
As expressed in Equation 1, null signals are assigned to the L reserved tones and the information signals are assigned to the remaining tones. Here, the L reserved tones (positions of the L sub-carriers) are previously appointed between the transmitter and the receiver in an initial transmission and do not change during data transmission.
The N-point IFFT unit 207 performs an N-point IFFT operation on the X(=N−L) input signals inputted from the tone assignment unit 205, and outputs the resulting IFFT operation values to a parallel to serial converter 209. The parallel to serial converter 209 converts the IFFT operation values from serial to parallel and outputs the resulting values to a gradient algorithm unit 211. The gradient algorithm unit 211 reduces the PAPR of the output signals of the parallel to serial converter 209 by using P-waveform that is produced by the L reserved tones. The output signal of the gradient algorithm unit 211 becomes a signal x+c produced by adding the output signal x of the IFF unit 207 to the signal c produced using the P-waveform. A detailed structure of the gradient algorithm unit 211 is illustrated in FIG. 3.
The conventional gradient algorithm will be described below with reference to FIG. 3.
Referring to FIG. 3, the gradient algorithm unit 211 includes a P-waveform generator 301, a peak detector 303, a position mover 305, a phase rotator 307, a scaler 309, a complex adder 311, a PAPR calculator 313, and a controller 315.
The P-waveform generator 301 generates the P-waveform having the impulse characteristic by using the reserved tones. As described above, the reserved tones are determined by randomly selecting one or more tones among the N tones, performing several hundred thousand to million times the operation of producing the impulse waveform, and selecting a combination of the tones having the most ideal impulse waveform.
The peak detector 303 detects a maximum peak of the signal x outputted from the parallel to serial converter 209. The position mover 305 moves the peak of the P-waveform to the position of the detected maximum peak. The phase rotator 307 matches a phase of the moved P-waveform with a phase of the maximum peak detected on a complex plane. The scaler 309 scales the P-waveform outputted from the phase rotator 307 depending on magnitude of the maximum peak, and then outputs the signal c. Here, the signal c is a calculated value that is optimized to remove the maximum peak of the signal x outputted from the IFFT unit 207.
The adder 311 outputs the signal x+c by complex addition of the signal x and the signal c, which are respectively outputted from the IFFT unit 207 and the scaler 309. The PAPR calculator 313 calculates the PAPR of the signal x+c outputted from the adder 311. The controller 315 compares the calculated PAPR with a predefined target PAPR. When the calculated PAPR is greater than the target PAPR, the signal x+c is fed back to the peak detector 303 so as to repeat the gradient algorithm. On the contrary, when the calculated PAPR is less than the target PAPR, the signal x+c is outputted as a transmit signal.
The above procedures are repeated until the PAPR of the transmit signal is below the predefined target PAPR. To prevent an infinite repetition, the system sets the maximum number of times of repetition. Accordingly, when the above procedures are repeated the set number of times, the signal is outputted even though the PAPR of the transmit signal is not below the target PAPR. For example, when the target PAPR is 8 dB, the repetitive procedure stops when the PAPR reaches 8 dB or below. When the procedure reaches the set number of repetition times, the signal corresponding to the last PAPR is outputted as the transmit signal.
Similarly, the conventional gradient algorithm searches the maximum peak among the time-domain sample values and reduces it to below the target level. If several peaks exceeding the target level occur, the number of times of the repetition of the algorithm increases. Consequently, computation complexity and processing time also increase.